Automatic telephone system



Jan. 20, 1931. R. F. STEHLIK 1,739,606

AUTOMATIC TELEPHONE SYSTEI Filed March 23. 1928 9 Sheets-Sheet 3 Inventor Ru l hF STihhk Jan. 20, 1931. R. F. STEHLIK 1,789,606

AUTOMATIC TELEPHONE SYSTEI Filed March 23, 1928 9 Sheets-Sheet 4 AIZ 5/2 772 iii-j.

Jan. 2-0, 1931. R. F. STEHLIK AUTOMATIC TELEPHONE SYSTEI Filed March 23, 1928 9 Sheets-Sheet 5 ii mmm I m @m 1 mkm 5 Ema Jan. 20, 1931. R. F. STEHLIK 1,78

AUTOMATIC TELEPHONE SYSTEM Filed March 25. 1928 s Sheets-Sheet 6 Ru h F 511% Jan. 20, 1931. R. F. STEHLI K 1,789,606

AUTOMATIC TELEPHONE SYS'I'EI Filed March 23. 1928 9 Sheets-Sheet 7 ii; mm

mm? HEN U M w EN m5 Nm Patented Jan. 20, 1931 UNITED STATES PATENT OFFICE RUDOLPH F. STEHLIK, OF CHICAGO, ILLINOIS, ASSIGNOR T RESERVE HOLDING COM- PANY, OF KANSAS CITY, MISSOURI, A CORPORATION OF DELAWARE AUTOMATIC TELEPHONE SYSTEM Application filed March 23, 1928, Serial No.

The present invention relates to improvements in or relating to automatic telephone systems and more particularly concerns the provision of a small private automatic exchange although it will be understood that features of the invention are capable of much Wider interpretation.

The arrangement described consists of a private automatic branch exchange having an extension to a main exchange and adapted to be operated solely by relays.

One of the objects of the invention is the production of a small automatic exchange operated solely by relays which would be much cheaper than those hitherto employed,

while a subsidiary feature is to provide a private or automatic branch exchange with an extension to a main exchange which is operated solely by relays and which is very 2:) cheap to install.

Among important features of the invention may be mentioned the provision of arrangements by which a call set up over one primary trunk may be transferred to another primary trunk if the call is a local call, but if there is no other trunk available then the call can be set up locally over the first primary trunk.

It is possible, according to the invention, to utilize a group of relays associated with the primary trunk to effect a call from a local subscriber to the main exchange, from a local subscriber to another local subscriber by transferring to another primary trunk, from a local subscriber to another local subscriber by the original primary trunk if there is no other trunk available and a call from a main exchange subscriber to a local subscriber. It will thus be appreciated that the group of relays associated with the first primary trunk contains a number of novel features in varying combinations, which consist of a. relay group capable of dealing not only with all the functions mentioned but dealing with two, three or more in varying combinations. Again, it will be appreciated that a second primary trunk circuit is provided which is capable not only of receiving an originating call but also of receiving and dealing with a call transferred to it. The

264,262, and in Belgium August 13, 1927.

transfer is eflected when a discriminating digit is dialled indicating that the call is not for the main exchange, no discriminating digit being required in the case of a call from the main exchange, and accordingly further features of the invention arise out of the fact that according to the route over which a group of relays is seized it is adapted to operate differently according as to Whether it is seized directly or seized after some series of impulses have already been transmitted.

Among further novel features may be mentioned a counting relay chain in which a single controlling relay, which energizes after the termination of one impulse and remains energized until the end of the next impulse, controls the operation of the chain of relays.

Another feature of the invention consists of relays which between them perform the functions of ringing, giving a busy tone and charging the local battery. These and other features of the invention will be better understood by referring to the accompanying description in which, Figs. 1 to 11 show diagrammatically the circuits by which various connections can be set up. Fig. 1 shows the line cut-off and switching relays of subscribers in the local exchange, Fig. 2 shows the battery feed group associated with the first choice primary trunk, Fig. 3 shows the battery feed group associated with the second choice primary trunk, Fig. 4 shows the relay recorder associated with the first primary trunk, Fig. 5 shows the relay recorder associated with the second primary trunk, Fig. 6 shows an alternative recorder to that shown in Fig. 4, Fig. 7 shows the battery charging, ringing, and busy arrangements, Fig. 8 shows an alternative of the busy arrangements.

Fig. 9 shows diagrammatically the general layout arrangement at the main exchange, that is the relationship between Figures 10 and 11.

Fig. 10 shows the repeater at the main exchange to which access is had from a subexchange, and from which access is had to the sub-exchange, while Fig. 11 shows a selector switch having access to the relay group from the normal type selector.

In order to follow the layout, the figures should be arranged as follows: Fig. 1 to the extreme left, Fig. 2 to the right of the upper portion of Fig. 1, Fig. 8 to the right of the lower-portion of Fig. 1, Fig. 4 to the right of Fig. 2, Fig. 5 beneath Fig. 3, Fig. 7 can be placed either above Fig. 2 or Fig. 3, according as to which battery feed relay group is being used, Fig. 10 should be placed on the left hand upper side of Fig. 2, while Fig. 11 should be placed to the left of Fig. 10, Fig. 6 is an alternative which can be substituted for Fig. 4.

Referring to the drawings the invention will be better understood by describing in the first place the setting up of a call assuming that both the battery feed groups are idle.

It will be assumed that subscriber No. 11 lifts his receiver with a view to establishing connection with subscriber No. 12, that is to say, it is desired to establish a simple local connection.

It should be mentioned that all the subscribers connected to the small private branch exchange have the first digit in their number as one. This is to distinguish them from subscribers connected to the main exchange, where the subscribers numbers al ways begin with a digit other than one. The first digit therefore discriminates between a call for a local subscriber and a call for a main exchange subscriber. lVhen subscriber N o. 11 lifts his receiver the following circuit is completed: earth, contact 21, subscribers loop, contact 22, line relay L11 to battery. The line relay energizes and completes the following circuit: earth, normally closed contact 23 of release relay R'Ll located in battery feed group G1, conductor 24, contact 25, contact 26, of relay LT S11, lower winding of relay OTS11 to battery. A second circuit is completed at the same time as follows: earth, normally closed contact 27 of relay RL2 of the battery feed group G2, contact 28, resistance 29, contact 30, contact 31 of relay ()TSll, lower winding of relay LTSll to battery. The resistance 29 is so chosen that in the circumstances described relay OTSll will energize before LTSll thereby opening contact 31 and preventing relay LTSll from energizing. Consequently the call is now extended from the subscribers line over contacts 32 and 33 of relay OTSll to the line relay LRO of the battery feed group G1. Line relay LRO accordingly energizes and at contact 34 completes a circuit for relay RLl as follows: earth, contact 34, contact 35 of relay LS, relay RLl to battery. Relay RL1 accordingly energizes and at contact 36 completes a circuit for the charging relay set shown in Fig. 7 for a purpose to be described later. Relay R-Ll also closes contact 37 and opens contact 23; the opening of this contact prevents the feed group G1 being taken into use by any other calling subscriber, at contact 37 relay R111 connects earth to the release trunk 38 so as to energize the cut-off relay CCll over contact 39 of relay OTSll. Relay R111 also provides a holding circuit for relay OTS11 from earth over contact 40, conductor 55, contact 56, relay OTSll to battery; this relay is therefore held energized independently of the initial energizing circuit. The cutoff relay CO11 on energizing opens contacts 22 and 21, whereby disconnecting line relay L11 which de-energizes and opens the circuits by which the feed groups G1 and G2 were originally tested and G1 seized. Further operations however take place in the battery feed. group G1. The energization of relays LRO and RLl jointly complete a circuit for relay US as follows: earth, contact 40, contact 41, normally closed conta ct 42, relay OS to battery. Relay OS energizes and disconnects the relay L121 from the conductors 43 and 44 extending to the main exchange at contacts 45 and 46 and connects up in place thereof the relay BB1 in the following circuit: conductor 43, contact 47, contact 48, contact 49, contact 50, the upper winding of relay BB1 to earth. The lower winding of relay BB1 is connected from battery over contact 51, contact 52 to conductor 44 so that battery feed is now connected from the relay group G1 to the conductors 43 and 44 to energize the line relay LRM at the main exchange (Fig. 10), which line relay is similar to relay LRl in that it has neither battery nor earth connections and is normally connected across the conductors and 44 so that it energizes in series with relay BB1.

The subscriber now commences to operate his calling device to send a series of impulses corresponding to the first digit. This digit only includes one impulse as the number dialled will be No. 12 and accordingly line relay LRO (lo-energizes once only. Relay OS at contact 53 closes a locking circuit for itself independent of contact 42 and contact 41 so that it is directly controlled by contact 40 and relay RLl. lVhen relay LRO dcenergizes it opens contact 49 so that battery ceases to be fed by relay BB1 over the conductors 43 and 44 to the main exchange and the line relay LRM thereat (lo-energizes. Relay LRO also closes at contact 57 a circuit for relay CH1 as follows: earth, contact 40, contact 57, contact 58, contact 71, contact 72, relay CH1 to battery. A branch of this circuit extends over conductor 59, contact 60 of relay F RB, normally closed contact 61 of relay FRC, winding of relay FRC, contacts 62, 63, 64, 65, 66 of the counting relays CRO, CR8, CR6, CR4, CR2 to relay CR1. Relays CH1, FRC and CR1 all energize. Relay CH1 on energizing completes a cirearth, conto battery.

contacts 62', 63, 6a, 65 and 66, relay CR1 to battery; contact (31 is opened but owing to the completion of the locking circuit over conductor 218 and contact 67 this is of no effect. Belay CH1 at contact 73 closes a circuit for its further encrgization independent of contacts 71 and 72, contact 72 being opened when relay 1 1C111 energizes. On the cessation of the first impulse relay LEO again energizes, contact 49 closes, reenergizing the line relay at the main exchange, and contact 3 1 closes to prevent reay llLl de-energizing. Relay 3L1 being slow to release does not de-energize on the momentary opening of the circuit. On the energization of relay LEO contact 57 is opened thus opening the circuit of relay CH1, which accordingly de-energizes after a short interval. Before this takes place, however, due to the energization of relay LRO after the first impulse a circuit is completed as follows: earth, contact 40, contact "ll, contact 76, conductor 219, contact 7 7 of relay FRC, upper winding of relay FEB to battery. Relay FEB energizes in this circuit, opens contact 60 and connects at contact 220 conductor 59 to relay FHA. As

however, conductor 59 is not again connected to earth, the energization of relay FEB in this case is without effect. \Vhen relay CH1 deenergizes, the following circuit for relay IS of the second battery feed group G2 is completed: earth, contact 10, contact 78, contact 79, conductor 80, contact 81, conductor 82, contact 83, conductor 84-, contact 85 of relay RL2 of the second battery feed group G2 and the upper winding f relay 1S to battery. This circuit is com pleted because the first digit only included one impulse and consequently relay CR1 was energized at the cessation of a train. If a train had consisted of more impulses it will be understood from the further description, that relay CR1 would not be energized and this circuit would not have been completed. The purpose of completing this circuit is that, now it is known that the call is for a local subscriber, it is possible to utilize a second battery feed group which has no access to the main exchange and so leaves the battery feed group G1, which has access t the main exchange, free for a further call. lt will be borne in mind that it is assumed that the battery feed group G2 is still not taken into use. On relay IS energizing the following circuit is completed: earth, normally closed contact 27, contact 86, conductor 87, relay LS of battery feed group G1 to battery. Relay LS accordingly energizes, locks itself energized at contact 88, and at contact 89 completes the following circuit: earth, contact 3 1-, contact 89, changeover lead 90, contact 91 of relay OlSll, lower winding of relay LTS11 to battery. Relay LT S11 accordingly energizes in this circuit and at contacts 92 and 93 connects the line relay LRL in circuit with the subscribers loop. Relay LS 011 energizing opens the circuit of relay R 1 at contact 35 which relay accordingly de-energizes and by opening contact 10 permits relays OS and LS to ale-energize. It also removes earth from conductor 55 and permits relay OTSll to de-energize. Relay CTSll accordingly disconnects the relay group G1 thereby permitting the relays thereat to restore to normal, including the relays FRC and CR1. Due to the sluggishness of the release of relay RLl, while this has been taking place the connection has been transferred and the relay LRL of the battery feed group G2 is energized over the subscribers loop, complet-ing a circuit for relay RL2 at contact 94: as follows: earth, lower winding of relay 1S, contactrelay l-l-L2 to battery. Relay RL2 accordingly energizes and opens contact 27, thereby opening the energizing circuit of relay LS, this occurring in the event of the relay RLl at the battery feed group G1 de-energizing before relay R 2 energizes. Generally speaking relay RL2 will energize first owing to the sluggishness of relay ltLl in falling away so that it is really the opening of the locking circuit of the relay LS at contact 27 which causes the de-energization of relay LS, as previously mentioned. Relay RL2 at contact 95 connects earth to the release trunk conductor 96, thereby maintaining cut-off relay C011 energized in the following circuit after earth has been removed at contact 37 from the release trunk conductor 38 in the group G1: earth, contact 95, conductor 96, upper winding of relay LTSll, contact 97, relay C011 to battery. As soon as relay RL2 energizes it completes a circuit for relay BBC2 as follows: earth, contact 95, contact 315, contact 114, relay l3BC2, over resistance 113 to battery. l3l3C2 energizes in this circuit and opens contacts 115, 116, 117 and 118 thereby disconnecting the speaking conductors from the calling subscriber during the transmission of impulses. Both relay LTSll and C011 are maintained energized in the locking circuit described above and everything is now in order for the subscriber to dial a second digit, in this case the digit 2. On the first impulse, when the subscribers loop is opened the relay LRL deenergizes and completes a circuit as follows: earth, contact 98 of relay RL2, contact 99 of relay BB2, contact 100, contact 101, relay CH2, to battery, while a branch circuit extends over the back contact 102 of relay SRB,

normal contact 103 of relay SRC, relay SRC, contacts 104, 105, 106, 107, 108 of relays SRO, SR8, SR6, SR4, SR2 and relay SR1 to battery. Relays SRC and SR1 energize in eries. Relay CH2 on energizing completes at contact 110 an energizing circuit for relay ACT-I2 which also energizes and at contact 111 completes a locking circuit for relays SRC and SR1 as follows: earth, conact 95, contact 111, contact 112, contact 109, relay SRC, contacts 104, 105, 106, 107, and 108, relay SR1 to battery. On the completion of the first impulse when relay LRL again energizes, the following circuit is com pleted: earth, contact 98, contact 123, contact 124, contact 625 of relay SRC, lower winding of relay SRB to battery. Relay SRB energizes and closes a locking circuit for its lower winding over contact 133 and as there are two impulses in the series which has been transmitted, relay CH2 and relay ACH2 remain energized. The second impulse accordingly commences and relay LRL again de-energizes and this time a circuit is completed for relays SRA and SR2 as follows: earth, contact 98, contact 123, contact 101, contact 125, contact 126, relay SRA, contact 127, 128, 129, of relays SR9, SR7, SR5, SR3 to relay SR2. Relays SR2 and .SRA energize in this circuit, and are locked energized in the following circuit: earth, contact 270 of relay ACT-I2, contact 268 of relay SR1, contact 131, relay SRA, contacts 127, 128, 129, 130 to relay SR2 to battery. it contact 132 a circuit is completed for the upper winding of relay SRB over the previously described circuit, including contacts 101, 123, 98 to earth. Relay SRB is maintained energized over its upper winding, on this circuit because relay SRA has opened contact 112 and thereby opened the locking circuit for relay SRC and relay SR1, and hence, opened the circuit for the lower winding of relay SRB. Relays SRC and SR1 accordingly de-energize. The locking circuit for relays SRA and SR2 is now however completed as follows: earth, contact 95, contact 111, contact 213 of relay SRC, contact 131, relay SRA, contacts 127, 128, 129 and 130, relay SR2 to battery. It will be clear that relay SR2 should notclose its front contact before relay SRA has opened contact 112, as it is important'that relay-SR3 should not energize, which it might do if relay SRA was slow in opening contact 112. This can readily be arranged by those versed in the art of relay design. Relay SR2 is accordingly energized and remains energized as there are no further impulses to be sent to select subscriber No. 12. As the relays SR1 and SR2 are successively energized, the test leads T11 and T12 of the subscribers Nos. 11 and 12 are successively tested. The relay SR1 was energized when the test lead was connected to earth over the upper winding of relay LTS11, and accordingly a circuit was completed as follows: earth, contact 95, release trunk conductor 96, upper winding of relay LTSll, contact 97, conductor T11, contact 134 Fig. 5), conductor 135, contact 136 of relay CH2, contact 137 of relay IS, contact 138 of relay CH2, relay BR2 to battery. Relay BR2 accordingly energizes and at contact 139 connects earth over conductor 100 to the busy tone circuit, while at contact 140 it prepares a locking circuit for itself, which however is of no elfect as the closure of contact 134 is only momentary, relay SR1 deenergizing at the beginning of the second impulse and relay SR2 energizing and closing contact 141. The closure of contact 141 by relay SR2 completes a test circuit for the line of subscriber No. 12. If this subscriber is busy at this time it will have earth connected to its test lead T12 and accordingly relay BR2 will be energized over contact 141 in a similar circuit to that described with relation to the closure of contact 134. As there are no more impulses the relay CH2 de-energizes after a short interval. Relay ACT-l2 which would normally de-energize after a short interval responsive to the deenergization of relay CH2, does not deenergize at this time, as it is locked energized in the following circuit: earth, contact 95, contact 142 of relay IS, contact 147, contact 146, relay ACH2 to battery. The de-energization of relay CH2 completes the looking circuit for relay BR2 as follows: earth, contact 95, contact 142, contact 140, contact 143 of relay CH2, relay BR2 to battery. Relay BR2 connects busy tone from conductor 159 over contact 144 so as to signal subscriber. No. 11 that the number he requires is engaged. In such a case the subscriber will then restore his receiver opening the circuit for the line relay LRL, which in turn opens the circuit for relay RL2, and the opening of contact 95 causes all relays which are dependent upon this circuit to de-energize, and the second battery feed group G2 and the second recorder restore to normal and become available for another call. Assuming, however, that the subscribcr No. 12 was not engaged then relay BR2 would not be energized by the closure of contact 141 and the following circuit would be completed when relay CH2 de-energized-earth, contact 95, con tact 145 of, relay CH2, lowerwinding of relay RR2, contact 137, contact 130, conductor 135, contact 141, test lead T12, relay C012 to battery. In this circuit relays RR2 and C012 energize and relay CC12 disconnects the relay L12 from across the called subscribers line so as to prepare a clear cir- 1 cuit for ringing. Ringing relay RR2 on energizing opens contact 114 and thereby permits relay BBC2 to d-e-energize so that it closes at its contact 148 a short circuit for the lower winding of relay BR2. Relay RR2 at contact 564 short circuits the lower winding of relay BB2 to battery, thereby rendering relay BB2 sluggish in operation, and unresponsive to alternating current. It also at contact 154 shunts the upper winding of relay BB2 over resistance 155 to assist in the above eli'ect. Relay BB2 however does not (ls-energize due to its lower 'inding being short-circuited as it is locked energized over its upper winding in the following circuit ringing battery, relay BB (Fig. 7), conductor 157, upper winding of relay BB2, contact 149, contact 2315, contact 95 to earth. At the same time the generator ringing current is connected to the called subscribers line in the following circuit: battery, contact 206, winding H of the transformer, conductor 161, contact 150, conductor 151, contact 115, which is now closed, since relay BBC2 is d-e-energized, conductor 119, contact 152, conductor B12, the loop of called subscriber No. 12, back to conductor A12, contact 153, conductor 121, contact 11?, conductor 156, contact 154, resistance 155 to earth. It will be noted that this ringing circuit is not completed until relay BBC2 deenergizes which can only take place a short time after its energizing circuit is opened. This gives ample time for relay C012 to energize and cut-oil the relay L12. Ringing continues until the called subscriber answers, when a circuit is completed for relay BB2 as follows: earth, upper winding of relay BB2, conductor 156, contact 11?, conductor 121, contact 153, conductor A12, subscribers loop, conductor B12, contact 152, conductor 119, contact 115, conductor 151, contact 150, conductor 161, winding H of the transformer, contact 206 of relay MB 7) to battery. Belay BB2 energizes in this circuit, opens contact 315 in the circuit of the upper winding of relay BB2, which accordingly de-energizes, closes contact 20? so as to short-circuit the small capacity condenser 208 through which busy tone can be connected to calling subseribers line, and closes contact 209 to complete the speaking circuit. The speaking circuit may now be traced as follows: starting from calling subscribers station No. 11 over the conductor A, contact 93, condenser 210, contact 20?, conductor 156, contact 117, conductor 121, contact 153, conductor A12 to subscriber No. 12 and back over conductor B12, contact 152, conductor 119, contact 115, conduct-or 151, contact 211, contact 209, condenser 212, contact 92, conductor B, back to snbscribers station No. 11. The battery of the talking circuit is fed on the calling subscribers side by the line relay LBL, which it should be noted is connected across the conductors A and B, while battery is fed on the called subscribers side, by the relay BB2, which is connected across the conductors 151 and 156. The subscribers can now carry on conversation, and when finished, the calling and called subscribers restore their receivers. On the calling subscriber restoring his receiver, the line relay LRL (ls-energizes and opens contact 94, which opens the circuit of the relays R112 and IS thereby causing them to de-energize. Belay R112 opens contact 95, thereby permitting relay ACH2 to de-energize. Belay ACH2 de-energizes and opens contact 111, thereby opening the locking circuit for relays SBA and SR2, which accordingly de-energize. The opening of contacts 153 and 152 opens the circuit of relay BB2, which de-energizes and consequently everything is restored to normal. Assuming however, that the calling subscriber hangs up his receiver after the called subscriber has hung up, then the relay BB2 de-energizes and opens the talking circuit at contacts 209 and 207, and at the same time closes contact 315, the energizing circuit for relay BBC2 being thereby completed from battery, relay BBC2, contacts 114, 315, and 95 to earth. This circuit is, however, not effective as relay BBC2 is short circuited in the following circuit on the deenergization of BB2; earth, contact 95, contact 142, contact 147, contact 146, contact 216, contact 214 to left hand terminal of winding of BBC2, winding of BBC2, contact 114, contact 315, contact 95 to earth. Accordingly, the circuit of relay C012 is not opened. It is therefore essential before the called party can be effectively released that the calling subscriber should hang up, which is of course usual practice in the principal automatic telephone systems.

It will thus be seen that the foregoing description covers the case where both the feed groups are idle and a subscriber of the private exchange desires connection with another local subscriber. Briefly the operation may be summarized as follows: when the subscribers calling device operated for the digits 1 and 2, the first digit determines that he wants a local connection and therefore the main exchange line 13, 44 is released, and at the same time the call is transferred to the second battery feed group G2. He then dials digit 2 and tests the line of subscriber No. 12, ascertains whether such subscriber is busy or idle and if busy receives the busy tone, and if idle rings his bell until he answers. On his answering the speaking connection is established and at the conclusion of the conversation everything is restored to normal.

It will now be assumed that subscriber No. 11 calls subscriber No. 12 and that the second battery feed group G2 is busy, having been taken into use for some other call. In such a case the release relay B112 will be energized, consequently when contact 81 is closed and the change-over relay CH1 de-energizes, a circuit is completed for relay ALS as follows earth, contact 40, contact 78, contact 7 9,

conductor 80, contact 81, conductor 82, contact 83, conductor 84, contact 231, conductor 232, relay ALS to battery. Relay ALS energizes in this circuit and completes a locking circuit for itself at contact 233; at the same time it opens contacts 48 and 51 and closes contacts 234 and 235. At contacts 47 and 234, it connects exchange conductor 43 to earth, and at contacts 52 and 235 it connects exchange conductor 44 over resistance 236 to battery; it also at contact 51 disconnects the relay BB1, consequently battery is continued to be served to the main exchange over conductor 44 which is therefore rendered busy, but as the selector at the main exchange is only stepped to the first level which is dead, no other switches are taken into use, simply the conductors 43 and 44, and the switches associated therewith at the main exchange are all rendered busy and inaccessible to other calls. Contact 49 it will be noted is now cut out of the impulsing circuit at contact 48 so that the next series of impulses cannot effect any further impulsing at the main exchange.

, Relay ALS at contact 237 also completes a circuit as follows: earth, contact 40, contact 238, upper winding of relay FD, contact 237, conductor 240, contact 239 of relay CR1, conductor 21-8, contact 68 of relay FRA, contact 69 of relay FRC, relay FRC contacts 62, 63, 64, 65, 66, relay CR1 to battery. Relay FD at present cannot energize as its winding is short-circuited over conductor 218, and contact 67, of relay ACHl. Relay ACH1 however will shortly ole-energize, depending on its sluggishness, as its circuit is opened at contact due to the de-energization of relay CH1, so that the contact 67 is eventually opened and the energizing circuit through the upper winding of relay FD becomes effective. Relay FD energizes, locks itself energized at contact 241 and opens contact 238. The open ing of contact 238 opens the circuit of relays FRC and CR1 thereby causing them to be deenergized so that the recording relays are restored to normal in readiness to receive the next series of impulses. Relay FD at contact prepares a locking circuit for relay ACHl, at contact 563 prepares the busy tone circuit over conductor 159, and at contact 242 prepares the test circuit as will be described later.

It will be noted that relay BB1 .de-energized on the energization of relay ALS, due to the contacts 48 and 51 being opened and accordingly a circuit is completed for relay BBCl over contacts 40 and 244, relay BBCl, resistance 245, to battery. Relay BBCl on energizing in this circuit opens contacts 246, 247, 248 and 249 so as to prevent any disturbance to existing connections which may be in use when the next series of impulses are transmitted and it will be clear that relay BBCl remains energized throughout the series of impulses. The de-energization of relay BB1 after the digit 1 has been dialled simply maintains BBCl energized. Everything is now in order for the dialling of the second digit, which in this case consists of two impulses. Relay LRO responds and opens contact 49, which has now no effect owing to the opening of contact 48. It also momentarily opens contacts 34 and 41, but these have no effect, in the first case due to the sluggishness of relay RL1, and in the second case due to contacts 77 and 42 being opened. However a circuit is closed over contact 57 for relay CH1 as follows: earth, contact 40. contact 57. contact 58, contact 71, contact 72, relay CH1 to battery. Relay CH1 energizes and at contact 73 completes a locking circuit for itself independent of contacts 71 and 72. At contact 70 relay CH1 completes a circuit as before for relay ACHl from earth, contact 40, contact 70, relay ACHl to battery, while at contact 250 a circuit is prepared for relay BRl as follows: battery, relay BRl, contact 250, contact 242, test conductor 243, which is successively connected to test conductors T11, T12, etc., as relays CR1, CR2 are energized. Relay ACT-I1 closes contact 67 and completes a locking circuit for itself at contact 74 as follows: earth, contact 40, contact 75 of relay FD, contact 74, relay ACHl to battery. Relay ACHl opens contact 72 thereby preventing the relay CH1 from energizing again after it has once de-energized, although of course it is locked energized throughout the train of impulses, over contact 73. The impulse conductor is also connected over contact 73 to conductor 59 over contact 60 and contact 61 to energize relay FRC and thence over contacts 62, 63, 64, 65 and 66 to energize relay CR1. this circuit and relay FRC locks energized in the circuit: earth, contact 40, contact 67, conductor 218, contact 63, contact 69, relay FRC, contacts 62, 63, 64, 65 and 66,

relay CR1 to battery. At the end of the first 1 impulse relay LRO again energizes and closes contact- 41, whereby a circuit is completed for relay FRB as follows: earth, contact 40, contact 41, contact 76, conductor 219, contact 77, upper winding of relay FRB to battery. Relay FRB energizes, opens contact 60 and closes contact 220 and at contact 231, con:- pletes a locking circuit for its upper winding. The next operation takes place when relay LRO de-energizes on the second impulse, .2

when a circuit is completed as follows: earth, contact 40, contact 57, contact 58, contact 73, relay CH1 to battery, thereby keeping relay CH1 energized, while a branch extends over conductor 59, contact 220, contact 221, relay i- FRA, contacts 222, 223, 224, 225, relay CR2 to battery. Relay FRA energizes and opens contact 68 thereby causing the relays FRC and CR1 to ole-energize; at contact 239 a circuit is completed for the lower winding of Relays FRC and CR1 energize in relay FBB, which therefore is maintained operated independently of contact 77 which opens when relay FBC de-energizes. Belay CB2 also energizes and opens contact 66, thereby again disconnecting relay CR1 and prepares a circuit over 226 for relay CB3. Belay BB1 1 locks energized in the following circuit: earth, contact 40, contact 67, conductor 218, contact 251 (relay FBC having tie-energized), contact 252, relay FBA, contacts 222, 223, 224, 225, relay CB2 to battery. Belays FBA and CB2 are therefore maintained energized, and as this is the final impulse of the series a test is made to ascertain whether the called line is busy or not. Assuming that the called line is busy then there will be an earth connection on conductor T12 as previously described. A circuit for busy relay BB1 will therefore be completed as follows: earth on conductor T12, contact 253, conductor 243, contact 242, contact 250, relay BB to battery. Belay BB1 therefore energizes and locks energized in the following circuit: earth on contact 40, contact 254, contact 255, relay BB1 to battery; this circuit being completed when relay CH1 de-energizes at the end of the series of impulsesi Belay BB1 thereupon connects earth to the busy start conductor 160 at contact 185 whereby a busy tone is received over conductor 159, contact 563 and contact 562 as will be described later. It passes through the condenser 256 to the A speaking conductor from whence it extends over contact 32 of relay OTSll to the calling subscribers telephone and informs him that the number he requires is busy. He thereupon restores his receiver causing LBC to ole-energize, which in turn causes the de-energization of relay BLl and the opening of contact 40, whereupon relays ALS, OS, ACHl, FD, BB1 and BBCl decnergiz also relays FBA and CB2 de-energize; FBB it should be mentioned de-energized on the cessation of the second impulse as its locking circuit over the upper winding was opened at contact 77 when relay FBC deenergized and its energizing circuit over the lower winding was opened at contact 57 when the line relay LBO energized on the termination of the second impulse. The removal of earth from the hold conductor 55, on the opening of the contact 40 brings about the release of relay OTSll, while the opening of contacts 37 and 39 enables relay COll to de-energize. Everything is now restored to normal and can be taken into use by another subscriber. If however the called subscriber had been idle then on the relay CH1 deenergizing a circuit would have been com pleted as follows: earth, contact 40, relay BB1. contact 2557', contact 242. conductor 243. contact 253, test conductor T12, relay C012 to battery. Belay BB1 and cut-01f relay C012 energize in this circuit. Belay BB1 at contact 565, short-circuits the lower winding of relay BB1 to battery; this renders relay BB1 sluggish in operation and unresponsive to alternating currents. It also at contact 262 shunts the upper winding of relay BB1, over resistance 263 to assist in the above effect. Belay BB1 further opens contact 244, thereby permitting relay BBCl to de-energize; it closes contact 258, thereby completing a circuit from earth and contact 40 over contact 258, contact 443, upper winding of relay BB1 to conductor 157 which is connected to battery. Ringing relay BB1 is therefore locked energized and at the same time ringing current is received back over Conductor 161, which passes over contact 259, contact 246 to the B speaking conductor, contact 260, conductor B12 through the bell at the station of subscriber No. 12, back over conductor A12, contact 261, contact 249, contact 262, resistance 263 to earth. On the deenergization of relay BBCl the lower winding of ringing relay BB1, is short-circuited at contact 264, but the relay is locked energized over contacts 443 and 258. The sub scribers bell. rings and on lifting his receiver in response thereto a circuit is completed for relay BB1 as follows: earth, upper winding of relay BB1, contact 249, contact 261, conductor A12, loop to a subscribers instrument, conductor B12, contact 260, contact 246, contact 259, to battery on conductor 161. Belay BB1 accordingly energizes and opens contact 443 to open the locking circuit of the upper winding of relay BB1 causing this relay to de-energize. Belay BB1 also closes contacts 265 and 266, whereby a speaking circuit is completed over the A and B speaking conductors. At contact- 267 the circuit of relay BBCl is shunted, to prevent this relay from again energizing after the called subscriber has answered. When the subscribers have finished talking and the calling subscriber restores his receiver, a release takes place substantially as previously described, the only difference being in this case that when the busy signal is received the relay BB1 does not require to be deenergized, nor does relay BBCl, otherwise the relays BLl, A CH1 and FD deenergize as before. Belay BB1 which serves to feed battery to the called subscribers loop, de-energizes due either to the called subscriber hanging up, if he hangs up first, or due to contacts 261 and 260 being opened on the de-energization of relay CB2, so that the battery feed group G1 is restored to normal independently of the called subscriber hanging up his receiver.

The next case to be considered is on the assumption that when subscriber No. 11 attempts to call subscriber No. 12 the first battery feed group G1 is already in use for another call. Under these conditions, when the line relay L11 pulls up no circuit will exist for energizing relay OTSll. The energizing circuit for relay OTSll it will be recollected Ill) passes through contact 23 which however is now open as relay RLl will be operated, relay llL'l being operated all the time the battery feed group G1 is in use. A circuit will therefore be completed for relay LT S11 as follows earth, contact 27 of relay R112 in battery feed group G2, contact 28 of relay 1S, resistance 29, contact 30, contact 31, lower winding of relay LTSll to battery. Relay LTSll energizes in this circuit and closes a locking circuit for itself over contact 97, which. locking circuit becomes effective when relay RL2 is energized, as will be described later. On relay LTS11 operating, it opens contact 26 as to prevent the energization of relay OTSll if battery feed group G1 should become vacant, closes contacts 92 and 93, thereby connecting relay LRL to the subscribers loop. Belay LRL accordingly energizes and at contact 94 completes a circuit for relay RL2 as follows: earth, contact 268 of relay SR1, contact 269 of relay IS, contact 94:, relay R122 to battery. Relay RL2 accordingly energizes and completes a locking circuit for relay LTSll as follows: earth, contact 95, conductor 96, upper winding of relay LTSll, contact 97, relay C011 to battery. Relay C011 energizes, disconnects the line relay L11 and earth at contacts 22 and 21 respectively and is held energized throughout the connection. Relay L11 deenergizes and opens contacts 25 and so as to open the original energizing circuit for re lay LTSll. Belay R112 closes contact 231, completing a circuit between conductors 232, and 84:, thereby preventing a call being transferred to group G2 as long as R112 is energized. Similarly it will be noted that IS cannot be energized through its upper wincing as long as RL2 is energized. Relay R112 at contact 162 connects earth to conductor 158, whereby, as described later, the arrangements for starting charging of the battery are prepared. It also closes contact 98 in readiness for the transmission of impulses. Subscriber No. 11 now operates his impulse sender to send a single impulse. Therefore relay LRL (lo-energizes and a circuit is completed as follows: earth, contact 98, contact 99, contact 100, contact 101, contact 102, contact 103, relay SEC, contacts 104-, 105, 106, 107, and 108 of relays SRO, SE8, SE6, S124 and SR2 respectively to relay R1 and battery. Relays SR1 and SEC accordingly energize and lock energized over the following circuit: earth, contact 95, contact 111, contact 112, contact 109, relay SRC, contacts 104, 105, 106, 107 and 108, relay SR1 to battery. A branch of the impulsing circuit extends from earth, contact 98, contact 99, contact 100, contact 101, relay CH2 to battery. Relay CH2 energizes and completes the following circuit for relay ACH2: earth, contact 95, contact 110, relay ACH2, to battery. Relay ACH2 energizes in this circuit, and closes contact 111 thereby preparing the locking circuit for relays SEC and SR1 as above desc 'ibed. At the end of the impulse, relay LRL again energizes and a circuit is completed as follows: earth contact 98, contact 123, contact 12 1, contact 625, lower winding of relay SR1 to battery. Relay SBB energizes and closes contact 133 to complete a locking circuit for its lower winding and also closes contact 125 and opens contact 102. As, however, there are no further impulses in the series, the energization of relay SRB actually brings about no other operation incidental to the closure of this contact. After a short pause, as there are no more impulses, relay CH2 (lo-energizes and after a further short pause relay ACH2 de-energizes. When this occurs it will be noted that the circuit for relay ltL2 is opened at contact 268, due to the energization of relay SR1, and at contact 270 due to the deenergization of relay ACH2. However, relay RL2 is held energized in the following circuit: earth, lower winding of relay IS, contact 9 1, relay RL2, to battery. Relay IS onergizes in this circuit, opens contact 269 to prevent its lower winding being again shortcircuited by either relay ACH2 or relay SR1, opens contact 28 and closes contact 86, which in this particular example is without any material effect, and closes contact 137. On the de-energization of relay ACH2, contact 111 is opened, which opens the locking circuit of relays SRC and SR1. Accordingly these relays de-energize and contact 268 is closed. This now, however, has no effect on short-circuiting the lower winding of IS as contact 269 is opened. Cu relay SRC de-energizing, contact 625 is opened and accordingly SRB deenergizes and the recorder restores to normal. Everything is now in readiness for the transmission of the nextdigit, which consists of two impulses. The first impulse brings about the de-energization of relay LRL and a circuit is accordingly completed as follows: earth, contact 98, contact 99, contact 100, contact 101, contact 102, contact 103, relay SEC, contacts 104, 105, 106, 107 and 108, relay SR1 to battery. Relays SEC and SR1 accordingly energize as before, SEC at contact 109 completing a locking circuit for itself over contact 111, relays CH2 and ACI-l2 being also energized, as previously described. Now, however, relay ACH2 is locked energized in the following circuit: earth, contact 95, contact 1412, contact 1 17, contact 1 1-.6, relay ACH2 to battery. The test circuit is now completed for relay BB2 as follows: battery, relay B112 contact 138, contact 137, contact 136, conductor 135, contact 13% to test conductor T11. Relay BB2 energizes but this is only for the duration of the first impulse. At the end of the first impulse, relay LRL again energizes and contact 124: is closed, whereby relay S313 is energized in a circuit already described. When relay LRL de-ener izes at the beginning of the second impulse, a circuit is completed for relay SBA as follows: earth, contact 98, contact 123, contact 101, contact 125, contact- 120, relay SBA, contacts 127,128, 129, 130, relay SR2 to battery. Belay SBA at contact 132 completes acircuit for the upper winding of relay SBB independent of contact 625 so that relay SRB is maintained energized in the following circuit: earth, contact 98, contact 123, contact 101, contact 132, upper wind ing of relay SB-B to battery. This circuit only lasts during the second impulse, however, and as soon as this impulse ceases relay SBB deenergizes. If the called. subscriber No. 12 were busy at this time, a circuit for relay BB2 would have been completed as follows earth on conductor T12, contact 141, conductor 135, contact 136, contact 137, contact 138, relay BB2 to battery. Belay BB2 would accordingly energize when the impulse ceased. On the de-energization of relay CH2 due to no further impulses being received, relay BB2 would lock energized in the following circuit earth, contact 95, contact 142, contact 140, contact 143, relay BB2 to battery. Belay BB2 by closing contact 139 and thereby connecting earth to conductor 160, would start the busy signalling arrangements, whereby busy tone would be received over conductor 159, pass through condenser 208, condenser 210, contact 93. to the calling subscriber No. 11, thereby indicating to him that the subscriber he requires is engaged on another call. If, however, the subscriber No. 12 is free, relay BB2 will not energize and when relay CH2 is tie-energized as described a circuit will be completed as follows: earth, contact 95, contact 145, the lower winding of relay BB2, contact 137, contact 136, conductor 135, con tact 141, conductor T12, cut-off relay C012 to battery. Relays C912 and BB2 acc0rdingly energize in this circuit: Belay C012 cuts off the line relay L12 and earth from the conductors extending to the station of sub scriber No. 12, while relay BB2 opens contact 114, thereby opening the energizing circuit of relay BBC2, which was completed as follows when relay BL2 energized; earth, contact 95, contact 315. contact 114, relay B1302, resistance 113 to battery. Belay BB2 also at contact 564. short-circuits the lower winding of relay BB2 to battery, and at contact 154 shunts the upper winding of relay BB2 to earth over resistance 155. This renders relay BB2 very sluggish in operation and unresponsive to alternating currents. The de-energization of relay BBC2. short circuits the lower winding of relay BB2 at contact 148. but this relay does not de-energize as it is locked energized in the following circuit: earth, contact 95, contact 315, contact 149, upper winding of relay BB2, to battery on conductor 157. The completion of this circuit, as will be described later, causes alternating current to be connected in series with battery on conductor 161 so that a ring ing circuit is completed as follows: battery, ringing current over conductor 161, contact 150, conductor 151, contact 115, conductor 119, contact 152, conductor B12, subscribers loop and subscribers station No. 12 through the bell of that station, conductor A12, contact 153, conductor 121, contact 117, conductor 156, contact 154, resistance 155 to earth, while a branch circuit extends through the upper winding of relay BB2 to earth. Binging continues until the called subscriber re moves his receiver and loops the speaking conductors whereupon a battery circuit is completed through the windings of relay BB2 causing this relay to energize. At contact 315 of this relay the circuit of the up per winding of relay BB2 is opened thereby causing this relay to de-energize, as its lower winding is short circuit-ed at contact 148. At contact 214 relay BB2 opens the circuit in which the relay BB02 is short-circuited during the ringing period. It closes contacts 207 and 209, thereby cutting out the condenser 208, and completing a speaking loop, battery being fed to the called subscriber over the windings of relay I'Vhen the subscribers have finished talking the calling subscrib er replaces his receiver and thereby allows relay LRL to de-energize. This is followed by the de-energization of relay B-L2 which in turn opens the circuit of relays CH2 and ACH2. Belay ACH2 opens contact 111, causing relays SBA and SR2 to de-energize. Relay SR2 on de-energizing opens contacts 152 and 153, permitting relay BB2 to de-energize and opens contact 141, permitting relay GU12 to de-energize. Everything is thereby restored to normal. t should be mentioned that the release is substantially similar in the case where the calling subscriber hangs up after receii ing the busy signal. It will be noted that this description is substantially identical with the description of the first arrangement in which the subscriber, after dialling the first digit one is switched over on the second group G2, except that in this case the dialling of the digit one as a first digit is necessary in order to operate the. relay IS, and bring the battery feed group G2 to the same condition as would be effected by the original energization of IS over the group G1.

It might be opportune at this time to consider the case where the first group G1 is busy but the subscriber No. 11 requires an exchange number. When he initiates his connection he is switched through to the group G2 exactly in the same way as described when he desired connection with subscribed No. 12 and the group G1 was busy, but in this case the first digit does not consistof one impulse but a number greater than one, for example four. In this case the first impulse brings about exactly the same operations as previously described, namely, at the beginning of the impulse relays SB1 and SBO are energized, relays CH2 and AOH2 are energized and remain energized throughout the train of impulses, while relay SBB is energized on the termination of the first impulse. On the beginning of the second impulse a circuit is completed as follows: earth, Contact 98, contact 123, contact 101, contact 125, contact 126, relay SBA, contacts 127, 128, 129 and 180, relay SR2 to battery. Belays SBA and SR2 energize in this circuit, relay SBA opens con tact 112, thereby permitting relays SBC and SBl to de-energize. Belay SB2 at contact 108 opens the circuit of relay SBl and at contact 271 connects up relay SR3 in its place. Relay SBA completes a locking circuit for itself as follows: earth, contact 95, contact 111, contact 213, contact 131, relay SBA, contacts 127, 128, 129, 130, relay SR2 to battery. At contact 132 relay SBA completes a circuit for the upper winding of relay SBB, contact 101, contact 123, contact 98 to earth for the duration of the second impulse, while at contact 272 a circuit is completed for relay BB2 as follows: earth, contact 95, contact 273, contact 272, relay BB2 to battery. Belay BB2 energizes and locks energized in the following circuit: earth, contact 95, contact 278, contact 274, relay BB2 to battery. Therefore relay BB2 remains energized irrespective of the number of impulses dialled. The subscribed, however, will not listen to the busy signal as his calling device is still in operation. On

. relay BB2 energizing, it opens contact 99.

On the cessation of the second impulse relay SBB de-energizes due to contact 101 being opened. and opens the locking circuit of relays SBA and SR2. On the beginning of the third impulse, relays SBC and SR3 are energized in the same way as relays SBC and SR1 were energized by the first impulse. Belay SBA does not energize and relays SBC and SB3 are locked energized over contacts 109 and 112 in the same circuit as for the first impulse. On the cessation of the third impulse, relay SBB is energized over contact 625 and its lower winding and at the beginning of the fourth impulse relay SBA, and relay SR4 which is now connected up at contact 275 in place of relay SR2, energize, and relays SBC and SR3 deenergize. At the end of the fourth impulse, relay SBB de-energizes and after a pause relay CH2 ole-energizes and opens contact 123. After a further short pause relay AOH2 (ls-energizes and opens contact 111 whereby relays SBA and SB4 de-energize and the relays are therefore all restored to normal. The subscriber without listening for the busy tone may continue to dial, but this is without effect as contact 99 is now open so that the first impulse cannot pass to the counting relays over the circuit: earth, contact 98, contact 99, contact 100,

contact 101, contact 102, contact 103, relay SBO, contacts 104, 105, 106, 107 and 108 to relay SBl to battery. It is clear therefore that no further impulses can be effective so that when eventually the subscriber It will be noted that when either the relay 9 BLl or the relay BL2 energizes, earth is connected to the conductor 158 leading to the ringing, busy and charging relay group BBC, shown in Fig. 7. Belay BL1 efl ects this by closing contact 86, while relay B112 effects it by closing contact 162. WVhen this occurs the relay OH is energized. Belay OH closes contact 163, and completes a circuit for relay CO, which, upon energizing completes a locking circuit for itself over contacts 165 and 168.

Nothing further, however, takes places until earth is removed from the conductor 158, whereupon relay OH de-energizes and in conjunction with relay OO completes acircuit for relay MB over contacts 166 and 164. Belay MB accordingly energizes and remains energized until either earth is again connected to charging conductor 158 due to a subscriber making a call, whereupon contact 164 is opened and relay MB de-energizes, or the high Voltage relay HVB, the circuit of which is closed over contact 167 of the relay MB, energizes. This high voltage relay is a relay which only energizes when the voltage of the battery reaches a predetermined voltage above normal when the battery is being charged, so that it would only energize when the battery is sufficiently charged and when it does so it opens contact 168, thereby opening the locking circuit for relay CO which in turn opens the circuit for relay MB. Thus it will be appreciated that in either of the two conditions, namely, of a subscriber calling, or of the battery being fully charged, charging of the battery will not take place. This arrangement has the advantage that charging never occurs while any party is engaged in conversation, while at the same time the battery is maintained fully charged in between conversations. Belay MB, as has already been indicated, is for the purpose of starting charging and a brief description of the charging arrangements will now be given.

It will be assumed in the first case that there is a direct current power supply available. In such a case the fuses (Z1 and (Z2 will be inserted and the charging circuit may then be traced as follows: positive pole of power supply connected to conductor 170, contact 171, fuse d2, conductor 172, retard coil 173 to positive pole of battery, while negative pole of battery is connected over fuse (Z1, conductor 174, contact 175 to conductor 176 connected to negative pole of power supply. If, however, the local supply is of alternating current, then it is necessary to provide a current rectifying arrangement and this is obtained by means of the polarized relay PR.

It will be noted that when relay MR energizes it closes contact 169, whereby a circuit is completed for relay PR as follows: earth, battery, resistance 177, contact 169, lower windings of the polarized relay PR to earth. These windings do not bring about any operation of the relay by themselves, but simply serve as polarizing windings. Another circuit is completed as follows: conductor 170, condenser 178, contact 179, upper windings of polarized relay PR, contact 180 to conductor 176. Alternating current, therefore flows through these windings and causes the armature of the polarized relay to alternately close contacts 181 and 182 in accordance with the frequency of the altenating current supply. Contact 181 is at present without effect but contact 182 connects direct earth over contact 183 of relay MR and the low tension winding L of the transformer to the negative pole of the exchange battery. In these circumstances, that is, when charging by alternating current, the fuses (Z1 and (Z2 are removed. The high tension winding H of a transformer is then connected direct across the conductors 170 and 176 of the power supply over contacts 171 and 184 respectively. This causes an alternating current of the same frequency and of suitable voltage to be generated in the low tension winding L of the transformer. Normally this would simply serve to ensure alternating current to flow through the battery. However by suitably designing the windings of relay PR, contact 182 is closed only for one half cycle namely the half cycle in which the direction of current is such as to charge the battery; during the other half-cycle, contact 182 is open and consequently no charge can take place. Consequently an efficient charging of the battery results.

It will be also noted that when either relay BB1 or relay BB2 energizes, earth is connected over contacts 185 and 139 respectively to the busy start conductor 160. The effect of this is to energize the relay 13 over contact 186 of relay RB. Relay 13 energizes and closes contact 187 to energize relay 12. Relay 12 energizes and at contact 188 closes a circuit for relay 11 and at contact 189 com- T pletes a circuit from earth on conductor 160 over contact 189 to busy tone relay BT, resistance 190 to battery. Relay BT energizes and by closing its contact 191 short-circuits itself so that it immediately tie-energizes and re-energizes intermittently, thereby causing the condenser 192 to be intermittently charged and discharged thus sending a tone over conductor 159. The relay 11 upon energizing closes contact 193 in parallel with contact 189 and at the same time closes contact 194. Contact 194 completes a short circuit about the relay 13, the resistance 195 preventing the battery being short-circuited. Relay 13 accordingly de-energizes after an interval as it is slow to release and is made additionally slow by having its winding short circuited. On releasing, it opens contact 187 to short-circuit at contact 196 the winding of relay 12. As make-before-break springs are provided in order to render this short circuit effective to reduce the time of release of relay 12, a resistance 197 is provided to prevent the battery being connected direct to earth. In a similar way, relay 12 when it releases opens contact 188 and closes contact 198, whereby the relay II is short-circuited. Resistance 199 is provided for the same purpose as resistance 197 that is to guard against short-circuiting the battery while contacts 188 and 198 are momentarily closed. For a while contacts 189 and 193 are opened and the intermittent operation of the relay BT ceases, thereby giving a pause in the busy tone, but as soon as contact 194 opens, relay I3 energizes and causes the re-energization of relay 12, which in turn causes the reenergization of relay II, then the relays successively release as previously described, and consequently the busy tone is given intermittently, the cycle of the operation of relays 11, 12, and 13 continuing as long as earth is connected to the busy start conductor 160. It will thus be seen that the connection of earth to the busy start conductor 160 causes the intermittent connection of a busy tone current to conductor 159 which conductor is insulated from all battery and earth connections by condenser 192.

It will have been noted that whenever either of the ringing relays R111 or BB2 are energized the upper winding of these relays is connected to conductor 157 which extends through the ringing relay RB to battery. WVhenever this occurs the relay RB energizes and at contact 200 completes a circuit for the relay 13, which in turn causes the operation of relays 12 and 11 in the manner previously described. These relays therefore, operate intermittently and while the closure of contacts 189 and 193 is without effect, seeing that there is no earth on the busy start conductor 160, the contact 201 is intermittently opened and closed. At the same time contact 202 of the relay RB completes a circuit over contact 203 through the upper windings of the relay PR, contacts 204, and 205 of relay 

